AM335X 使用外部时钟芯片，32.768K晶振要不要接？

大家好：

        1、AM335X 使用外部时钟芯片PCF8563，不用内部的RTC，但是要用PMIC_POWER_EN控制TPS65910输出，32.768K晶振要不要接？不接会有什么影响？

        2、如果不用外部唤醒，EXT_WAKEUP能悬空吗？

        我看官网开发板，RTC-Only-Mode ,CAP_VDD_RTC接1uF电容到地，但是PDF上“OSC1 Start-up Time”显示，CAP_VDD_RTC

    电压是高电平后，晶振才能起振啊，但是CAP_VDD_RTC接1uF电容到地，不能拉高了吧，那外接的32.768K的晶振有什么作用？求解惑！

Eggsy Pang：

这里有你所有的答案：

http://processors.wiki.ti.com/index.php/AM335x_Schematic_Checklist

External Interrupt (EXTINTn)

This signal is active high for PG1.0 and active low for PG2.x. Boards designed to support all silicon revisions may want contain population options for both in case you move between different revisions of the AM335x during development. New designs are expected to use only PG2.1.
This signal connects directly to the Cortex A8 interrupt controller, which as a result makes this a level sensitive pin. It is recommended to consider using a GPIO signal instead of EXTINTn. The GPIO pins offer more flexibility with respect to polarity as well as the ability to be edge triggered.

Ethernet

While no series resistors are required for MII/RMII/RGMII, it is prudent include zero-Ohm stuff options for the TX and RX lines. Ideally, these option resistors should be as small as possible (0402 or smaller recommended) and should be placed as close to the transmitter as possible.

RTC

The following table describes what to do with each pin related to RTC functionality. Three use case scenarios are provided:

RTC-only mode: If you will be using the low power RTC-only mode. This use case allows low power operation of the AM335x by allowing only the RTC power supply to be ON while all the remaining supplies are OFF.
RTC timer functionality but no RTC-only mode: If you will be using the RTC feature but do not need RTC-only mode. This use case allows you to use the Real Time clocking features (eg, keeping time), but you do not need to support the low power RTC-only mode.
RTC feature disabled: If you will never use the RTC features. In this use case, the RTC functions are fully disabled.

Pin
Function
RTC-only mode
RTC timer functionality but no RTC-only mode
RTC feature disabled

VDDS_RTC
1.8 V power supply
Always on RTC 1.8 V power supply
any AM335x 1.8 V power supply3)
any AM335x 1.8 V power supply3)

CAP_VDD_RTC
RTC core voltage input/LDO output1)
1 uF decoupling capacitor to VSS
VDD_CORE4)
VDD_CORE4)

RTC_KALDO_ENn
Internal LDO enable input
VSS
VDDS_RTC
VDDS_RTC

RTC_PWRONRSTn
RTC power on reset input
1.8 V RTC power on reset2)
1.8 V PWRONRSTn 5)
VSS

PMIC_POWER_EN
PMIC power enable output
PMIC power enable input
No Connect
No Connect

EXT_WAKEUP
External wakeup input
1.8 V wakeup event signal
VSS
VSS

Datasheet power up sequencing
Figure 4-1, 2, 3
Figure 4-4
Figure 4-5

Note

lvsetianya1：

回复 Eggsy Pang:

非常感谢您的及时答复！这个文档我看过了，我想用PMIC_POWER_EN控制电源芯片使能输出，只能用“RTC-Only-Mode”，但是不用内部RTC时钟，我通过IIC外接了一个时钟芯片。RTC_XTALIN 和 RTC_XTALOUT 脚怎样接呢？是下图checklist文档里的那种情况？内部RTC没有，为什么还要外供一个32.768k的时钟呢？

Jian Zhou：

回复 lvsetianya1:

这种情况下，所有相关引脚按照check list去连，但是不需要加32K的时钟晶体。

lvsetianya1：

回复 Jian Zhou:

截图是checklist里关于内部RTC时钟没有使用的说明，第一条，为什么内部RTC时钟没有使用，RTC_XTALIN脚还要接一个1.8 volt LVCMOS clock source？

有什么作用？

Jian Zhou：

回复 lvsetianya1:

那个是指不用RTC内部的振荡器，而不是不用内部RTC。

如果确定你不用内部RTC，不用接32K晶体。